Ceruleum is a glowing blue energy source discovered and used by the Garlean Empire. Initially it was used as a fuel to burn to keep fireplaces lit and provide heat in the intensely frigid climate of Ilsabard. However, following the discoveries made by Solus Galvus, Ceruleum was found to be a potent fuel that could greatly enhance the power of their Magitek, from weapons to transport to the various devices used for everyday life.
It is no overestimation to say that the discovery of the efficacy of Ceruleum as a fuel source changed the course of the Garlean Empire forever, allowing them to begin their swift conquest of two of the Three Great Continents.
Ceruleum is abundant on the continent of Ilsabard, but only shows up in remote portions of Eorzea. It has been farmed out of Ilsabard and only remains in a few places.
As Ceruleum is incredibly volatile, it must be handled with extreme care. Any facility that houses Ceruleum is heavily guarded to prevent entry by any party that sought to misuse it, or any Voidsent like Bombs and Grenades that are atracted to it. The effect of Ceruleum igniting and exploding could be witnessed in Mor Dhona over Silvertear Lake, wherein the Ceruleum tanks reacted to the damage caused by Midgardsormr and erupted, turning the entirety of the lake to crystal.